A loudspeaker for audio signals converts electrical energy into mechanical energy by vibrating a diaphragm. The vibrating diaphragm causes motion of the surrounding air, thus producing acoustic energy.
The most common form of diaphragm for low frequency and midrange loudspeakers is a cone. A voice coil immersed in a magnetic field is attached to the neck of the cone, and the cone is attached to a rigid frame by means of flexible members at both the neck and the outer edge of the cone. These flexible members (i.e. a “spider” at the neck, and a “surround” at the outer edge) allow the cone to move fore and aft within the frame while maintaining axial alignment (i.e. preventing the cone from tilting or rocking).
In order to maximize the efficiency of the loudspeaker, and also extend its frequency range upward, the cone should have as little mass as possible. In order to reduce distortion, the cone should be as rigid as possible. In order to achieve high output capacity, the cone should have high mechanical strength. In order to move a high volume of air, the cone should be as large as possible within the given frame size. In order to achieve high output at low frequencies the cone assembly needs to be capable of long excursions.
Conventional loudspeakers are limited in the amount of acoustic energy that can be produced with acceptably audio low distortion. The limit arises primarily from the structure of the surround, which limits the excursions of the cone, and the stiffness of the cone, which distorts under large excursions.
The prior art offers different solutions to stiffening the cone. By way of example, U.S. Pat. No. 2,513,171 issued to J. A. Hassan, Jun. 27, 1950 discloses methods of stiffening a loudspeaker cone using struts extending from the voice to the outer edge of the cone.
U.S. Pat. No. 2,641,329, issued to S. Levy & A. Cohen, Jun. 9, 1953 deals with methods of stiffening a loudspeaker cone by attaching “transversely arched perforated plates bridging the central area of the diaphragm member” or by attaching a “horn shaped member secured at its smaller end to the junction of the dome cap and the diaphragm or directly to the dome cap, and secured at its larger end to the side of the diaphragm at a substantial distance from the smaller end.
U.S. Pat. No. 4,115,667, issued to B. Babb Sep. 19, 1978 discloses a method of stiffening a loudspeaker cone by means of a plurality of ribs extending along the surface of the dust cap and the surface of the cone.
U.S. Pat. No. 4,373,609 issued to C. D. Miller, February 1983, deals with a method of cone stiffening employing “a plurality of stiffener columns positioned in a plane to form a polygon essentially perpendicular to a central axis of the diaphragm with the ends of the columns being attached to the diaphragm intermediate the small end and the large end of the diaphragm.
Each of the aforementioned patents discloses a method of stiffening a cone which adds considerable mass to the cone, thus lowering the efficiency of the loudspeaker and reducing it's high frequency response. Accordingly, there is a need for a loudspeaker design which provides for delivering high volumes of air while simultaneously providing high electrical to acoustic efficiency, high frequency response and low distortion within a specified frame size.